The CDMA 1x/EVDO mobile system is a hybrid system that has evolved from the CDMA2000 system. EVDO stands for Evolution Data Only or Data Optimized and as suggested by this, is a data only system. One advantage of EVDO systems is that, it allows a high transfer rate for data. Another advantage of 1XEVDO System operating in hybrid mode is the ability to receive circuit switched services, such as voice calls, even during active data session in EVDO System.
Mobile devices typically include a list, referred to herein as a preferred roaming list, which indicates to the mobile device the preferred networks that the mobile device should be connected to. A preferred roaming list stores information about various networks within a geographic location and indicates to the mobile device which network it should connect to preferably. If a mobile device does not connect to a preferred network but connects to a less preferred network on the preferred roaming list, the mobile device includes a timer which, at the expiration thereof, triggers a rescan on the mobile device to try to acquire a more preferred network. When this is done, the mobile device drops the current network that it is connected to in an attempt to reconnect to a more preferred network.
The dropping of a network is problematic when the mobile device is operating in a CDMA 1x/EVDO system with a traffic channel established. EVDO hybrid systems require that the mobile device first acquire the CDMA system and wait for the CDMA system to be in an idle state before then acquiring the EVDO network. Once the EVDO network is acquired, data is transmitted to the mobile device through the EVDO network and voice and other circuit switched services such as short message service (SMS) use the CDMA system.
In the EVDO hybrid system, the CDMA 1x system takes precedence over the EVDO system. Thus, if ever the CDMA 1x connection is lost, the mobile device will drop any EVDO connection that is established and will attempt to reacquire the CDMA system. The problem with this is if the mobile device is not in a preferred system, the EVDO traffic channel may be dropped at periodic intervals when the best service rescan timer expires. This produces undesirable results since it results in lost data which needs to be resent, thus using up network resources. if the user is paying on a per packet basis, will also cost the user money. It further requires battery resources to establish a traffic channel again once the CDMA 1x system is acquired and the EVDO system is acquired.